The evolutionary history and ecological differentiation of the genus Exiguobacterium was characterized within natural communities from the Cuatro Cienegas Basin, Mexico. Exiguobacterium comprises both halophilic and alkaliphilic bacteria that are abundant among the aquatic systems of the Cuatro Cienegas Basin. We obtained complete sequences of the 16srRNA gene and partial sequences of four housekeeping genes (citC, rpoB, recA and hsp70) in 183 Exiguobacterium isolates retrieved from distinct aquatic systems. We defined three main phylogroups that are closely related to marine and thermophilic species of the genus. These phylogroups were neither specific to a given aquatic system nor to a particular salinity. Phylogenetic reconstruction indicated the presence of several small clusters within the phylogroups. These clusters consisted of isolates predominantly retrieved from sediment or water. Unifrac and AdaptML analyses confirmed this observation, pointing towards a clear pattern of differentiation linked to either sediment or water habitats. Our results are in line with the concept that niche differentiation is one of the main factors shaping prokaryotic populations and leading to evolutionary divergence.

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Lake San Lorenzo is one of the lakes at the NW karstic plain of the “Parque Nacional Lagunas de Montebello”, Chiapas, Mexico, that currently have eutrophic, turbid waters. To understand the recent history of this lake, multi-elemental contents determined by X-ray fluorescence analysis, organic matter and carbonate contents -determined by loss on ignition-, Corg/N ratios and diatom analyses from a 41 cm-long sedimentary sequence, were performed. The chronology of the sequence was established by one 14C date from the basal sediments, which allowed establishing that the studied record began in 1956. These new data are consistent with previously published analysis of a profile from the nearby Lake Balamtetik, that could not be dated by 14C, therefore this work provides a better chronological resolution of events. The diatom community along the Lake San Lorenzo sequence was dominated by high nutrient tolerant species, which allows inferring that the lake was already turbid and eutrophic since 1956. Erosion indicators (titanium and magnetic susceptibility) allowed identifying two episodes of basin-wide erosion associated with long distance sediment transport to the lake (very likely through the ”Río Grande de Comitán”) the first episode from 1970 to 1985 and the second one from 1990 to 2000.

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